US 20050141649 A1 Abstract An estimation apparatus for estimating a channel response of a radio propagation path using a received signal including a first known signal, comprises a generator which generates a reference signal matrix, a calculator which calculates a generalized inverse matrix of the reference signal matrix including singular values which exceed a preset threshold value, an estimation unit configured to estimate an impulse response of the radio channel modeled using a transversal filter based on the first known signal and the generalized inverse matrix, and a converter which converts the estimated impulse response into a frequency-domain signal to acquire a frequency transfer function of the channel.
Claims(11) 1. A method for estimating a channel response using a received signal, the method comprising:
generating a reference signal matrix; calculating a generalized inverse matrix of the reference signal matrix including singular values which exceed preset threshold value; estimating an impulse response of the channel modeled using a transversal filter based on a known signal included in the received signal and the generalized inverse matrix; and converting the estimated impulse response into a frequency-domain signal to acquire a frequency transfer function of the channel. 2. The method according to measuring a carrier-to-noise ratio of the received signal; and setting the threshold value based on the carrier-to-noise ratio. 3. The method according to estimating a modulation and coding scheme of the received signal; and setting the threshold value based on the estimated modulation and coding scheme. 4. The method according to measuring a carrier-to-noise ratio of the received signal; estimating a modulation and coding scheme of the received signal; and setting the threshold value based on a selected one of the measured carrier-to-noise ratio of the received signal and the estimated modulation and coding scheme. 5. The method according to the received signal is an OFDM signal, the generalized inverse matrix is given by where A represents the reference signal matrix, (A ^{H}A)^{31 1}A^{H }the generalized inverse matrix, λi the singular value of the reference signal matrix A, w_{i }the M-dimensional singular vectors, v_{i }the L-dimensional singular vectors, M the number of subcarriers of the OFDM signal, L the number of paths included in the modeled channel, k the positive integer higher than 0 and lower than L. 6. An apparatus for estimating a channel response using a received signal , the apparatus comprising:
a generator which generates a reference signal matrix; a calculator which calculates a generalized inverse matrix of the reference signal matrix including singular values which exceed a preset threshold value; an estimation unit configured to estimate an impulse response of the channel modeled using a transversal filter based on a first known signal included in the received signal and the generalized inverse matrix; and a converter which converts the estimated impulse response into a frequency-domain signal to acquire a frequency transfer function of the channel. 7. The apparatus according to a measuring unit configured to measure a carrier-to-noise ratio of the received signal; and a setting unit configured to set the threshold value based on the carrier-to-noise ratio. 8. The apparatus according to an estimating unit configured to estimate a modulation and coding scheme of the received signal; and a setting unit configured to set the threshold value based on the estimated modulation and coding scheme. 9. The apparatus according to a measuring unit configured to measure a carrier-to-noise ratio of the received signal; an estimating unit configured to estimate a modulation and coding scheme of the received signal; and a setting unit configured to set the threshold value based on selected one of the measured carrier-to-noise ratio of the received signal and the estimated modulation and coding scheme. 10. A receiver apparatus for use in an OFDM radio communication system, comprising:
a receiving unit which receives an OFDM signal of the RF band and generates a received signal including a first known signal; a generator which generates a reference signal matrix; a calculator which calculates a generalized inverse matrix of the reference signal matrix including singular values which exceed a preset threshold value; an estimation unit configured to estimate an impulse response of the radio propagation path modeled using a transversal filter based on the first known signal and the generalized inverse matrix; a converter which converts the estimated impulse response into a frequency-domain signal to acquire a frequency transfer function of the radio channel; and a demodulator which demodulates the received signal based on the frequency transfer function. 11. The receiving apparatus according to where A represents the reference signal matrix, (A
^{H}A)^{31 1}A^{H }the generalized inverse matrix, λi the singular value of the reference signal matrix A, w_{i }the M-dimensional singular vectors, v_{i }the L-dimensional singular vectors, M the number of subcarriers of the OFDM signal, L the number of paths included in the modeled channel, k the positive integer higher than 0 and lower than L.Description This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2003-433640, filed Dec. 26, 2003, the entire contents of which are incorporated herein by reference. 1. Field of the Invention The present invention relates to a channel response estimation apparatus for estimating a channel response in an OFDM radio communication system, and a receiving device using the channel response estimation apparatus. 2. Description of the Related Art In high speed radio communication systems, compensation for signal waveform distortion due to multipath delay waves has become indispensable. As a modulation scheme for use in radio communication, orthogonal frequency division multiplexing (OFDM) is known which utilizes a plurality of carriers and each carrier is orthogonal to others (called sub-carriers). In OFDM, inter-symbol interference can be avoided by converting an OFDM signal into a cyclic signal using guard intervals. This prevents signal waveform distortion due to multipath delay waves, without an equalizer. In an environment in which signal waveform distortion occurs because of multipath delay waves, subcarriers of an OFDM signal are received with different gains and phases. To demodulate OFDM signal in such environment, it is necessary to estimate the frequency transfer function of the channel and perform demodulation based on the estimation result. In OFDM, in general, a predetermined known signal, which is already known to a transmission side and reception side, hereinafter called “known signal” is transmitted to estimate the frequency transfer function of a channel. In a method in which known signals are carried by all subcarriers in each OFDM symbol, it is not necessary to perform frequency-domain interpolation, which is effective when the range of variations with time in the response of the channel is small. Further, a scheme is well known in which the impulse response of a channel is estimated from such a known signal, and the estimated impulse response is subjected to Fourier transform, thereby estimating the frequency transfer function of the channel. Estimation of an impulse response is realized by, for example, modeling a channel using a transversal filter model, and estimating the tap coefficient of the filter. Jpn. Pat. Appln. KOKAI Publication No. 2003-124857 (document 1) discloses a method for estimating an impulse response using the least-square method. Jpn. Pat. Appln. KOKAI Publication No. 2003-32217 (document 2)discloses a method for estimating an impulse response using the mean square error method. In OFDM, in general, to reduce the interference between adjacent channels, several subcarriers located at the both edges of whole frequency band of an OFDM signal cannot be used for communication. This degrades the time-domain resolution. Accordingly, the method proposed by document 1 cannot separate delay waves that reach adjacent taps. In the method proposed by document 2, the estimation performance is significantly influenced by the initial value of the reverse matrix of a correlation matrix used in the recursive least square (RLS) method. It is an object of the invention to provide a channel response estimation apparatus capable of estimating, with high accuracy, a frequency transfer function as a channel response even if several subcarriers located at the both edges of whole frequency band of an OFDM signal cannot be used, and to provide a radio receiving device utilizing the estimation apparatus. In accordance with an aspect of the invention, there is provide an apparatus for estimating a channel response of a radio propagation path using a received signal including a first known signal, comprises a generator which generates a reference signal matrix, a calculator which calculates a generalized inverse matrix of the reference signal matrix including singular values which exceed a preset threshold value, an estimation unit configured to estimate an impulse response of the radio channel modeled using a transversal filter based on the first known signal and the generalized inverse matrix, and a converter which converts the estimated impulse response into a frequency-domain signal to acquire a frequency transfer function of the channel. Embodiments of the invention will be described in detail with reference to the accompanying drawings. (First Embodiment) Referring first to The modulated signals are sent to an inverse fast Fourier transformer (IFFT) The output signal of the D/A converter On the other hand, as seen from The output signal of the frequency converter The fast Fourier transformer The output signal of the fast Fourier transformer In general, in the above-described OFDM radio communication system, communication is performed without using some subcarriers located at the both edges of whole band of an OFDM signal, as shown in In the OFDM receiver shown in To correctly demodulate the signals, carried by the subcarriers, by the demodulator It is generally known that the frequency transfer function of a linear system is acquired by computing the Fourier transform of the impulse response of the system. Accordingly, the impulse response is estimated by acquiring an approximate channel using a transversal filter model with a tap terminal T as shown in Specifically, an OFDM transmitter transmits, at predetermined timing, an predetermined OFDM signal (the known signal for channel response estimation) including symbols known to an OFDM receiver. The OFDM receiver uses the transmitted signal to optimize the tap coefficient of the transversal filter as shown in The channel response estimation unit The signal output from the receiving unit The coherent combiner The generalized inverse matrix arithmetic unit The impulse response estimated by the impulse response estimation unit In Subsequently, the impulse response is estimated by applying the LS method to the received signal r(n) output from the coherent combiner The serial/parallel converter In equation (5), (A The calculation of equation (5) is performed by the matrix product arithmetic unit It is understood from the above that if a certain singular value λi is much lower than the other singular values λ To avoid this, in the embodiment, when the reference signal matrix generator In the equation (8), The thus estimated impulse response is subjected to frequency-domain conversion by the frequency-domain converter As described above, the embodiment enables an OFDM communication system, which cannot use some subcarriers of an OFDM signal, to prevent degradation of impulse response estimation accuracy, and enables the frequency transfer function of the channel to be estimated with high accuracy. (Second Embodiment) As shown in As described previously, degradation of the estimation accuracy of the transfer function can be prevented by acquiring a generalized inverse matrix, instead of using singular values not higher than threshold value TH, and singular vectors corresponding to the singular values. The optimal threshold value TH depends upon CNR. For instance, if threshold value TH is set too much low, the advantage of avoiding the noise enhancement. On the other hand, if threshold value TH is set too much high, distortion of a signal component is increased, therefore the estimation accuracy is degraded, although the noise enhancement is reduced. In light of this, threshold value TH set by the threshold value setting unit (Third Embodiment) As seen from In general, in a system that performs communication with MCS appropriately varied, different CNRs are employed between different MCSs. For example, a modulation scheme that uses s large multi-value modulation number is used in an environment in which CNR is high, while a modulation scheme that uses a small multi-value modulation number is used in an environment in which CNR is low. The same can be said of an encoding scheme. An encoding scheme of a high encoding rate is used in a high-CNR environment, and an encoding scheme of a low encoding ratio is used in a low-CNR environment. From this, it is expected that when communication is performed with a modulation scheme that uses a large multi-value modulation number, CNR is generally high. In light of this, in the embodiment, threshold value TH can be set to an optimal value according to the environment of the channel simply by changing threshold value TH by the threshold value setting unit (Fourth Embodiment) The optimization of threshold value TH may be performed by combining the manner of changing TH in accordance with CNR and the manner of changing TH in accordance with MCS. Referring now to On the other hand, if the first mode is not set, it is determined whether the second mode in which threshold value TH is changed in accordance with MCS (step S After determining threshold value TH, approximation of a generalized inverse matrix is performed without using a singular value lower than threshold value TH (step S Subsequently, the approximated generalized inverse matrix as expressed in equation (8) is substituted into equation (5), thereby acquiring the impulse response As described above, in the embodiment, threshold value TH is set based on parameter CNR or MCS selected by a user, thereby approximating a generalized inverse matrix using the set threshold value. (Fifth Embodiment) A fifth embodiment of the invention differs from the first embodiment in that in the former, the OFDM transmitter performs space multiplexing, and a plurality of transmission antennas transmit OFDM signals. When transmitting OFDM signals using space multiplexing, it is necessary to estimate the frequency transfer function of each channel and at least one receiving antenna of an OFDM receiver. The sequence composed of OFDM symbols LP The OFDM symbols LP The use of the impulse response The present invention enables even an OFDM radio communication system, in which all subcarriers in the bandwidth range cannot be used, to accurately estimate a frequency transfer function as a channel response, and to accurately demodulate a received OFDM signal using the estimated frequency transfer function. Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. Referenced by
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